1. Field of the Invention
The present invention relates to a converter, particularly to a converter which is applied to a taking lens for a TV camera.
2. Description of the Related Art
TV screens with aspect ratios of 16:9 and 4:3 are used, and an aspect ratio switchable camera which can switch the aspect ratios has been manufactured.
The aspect ratio switchable camera has a solid-state imaging device with a 16:9 rectangular image area (an effective image area) surrounded by a xcfx8611 image circle, which is 11 mm in diameter. In a 16:9 mode, the camera uses the entire effective image area. In a 4:3 mode, the camera electrically masks areas at both sides of the effective image area to use only a 4:3 imaging area within the effective image area.
When the camera switches from the 16:9 mode to the 4:3 mode, a view angle reduces by approximately 20%. Thus, the camera does not show the full performance of the taking lens, and the maximum view angle is reduced. For the aspect ratio switchable camera, a taking lens in which a ratio converter that has a mode switching function with a magnification of 0.8 (9/11) has been known (Japanese Patent Application Laid-open Nos. 8-171043 and 11-101933). In the ratio converter, a ratio conversion lens with an afocal magnification of 0.8 is inserted into an optical system when the camera switches to the 4:3 mode, so that an image formed within an xcfx8611 image circle is reduced to an image formed within a xcfx869 image circle.
On the other hand, a variety of types of auxiliary lenses have conventionally been marketed such as a wide-angle conversion lens and a tele conversion lens which can be easily attached and detached to and from the front of the taking lens. The wide-angle conversion lens reduces the focal length, and also forms an image formed on the 16:9 image area within the xcfx8611 image circle without vignetting.
However, since the ratio converter lens is usually provided to a tele converter together with the tele conversion lens and the like, it is difficult to detach the ratio conversion lens. Therefore, the ratio conversion lens is exclusively used for one taking lens, and cannot be shared by multiple taking lenses.
In contrast, the auxiliary lens such as the wide-angle conversion lens described above can be easily attached and detached to and from the front of the taking lens, and thus it can be shared by multiple taking lenses. By applying this advantage of the auxiliary lens, a wide-angle conversion lens with an afocal magnification of 0.8 on the market is considered to be used as a ratio conversion lens. More specifically, the wide-angle conversion lens with the magnification of 0.8 is used as a ratio conversion lens to expand the view angle; as a result, the image formed on the xcfx8611 image circle for the 16:9 mode is reduced to the image formed on the xcfx869 image circle for the 4:3 mode.
Still, the wide-angle conversion lens forms an image in the imaging area without vignetting in a state where the image size of the imaging area is constant (xcfx8611), hence the wide-angle conversion lens is so designed as to guarantee the image size of xcfx8611 even though it is attached to the taking lens. On the other hand, the ratio conversion lens is satisfiable as long as the image size xcfx869 of the imaging area at the time of the 4:3 mode is secured. Because of that, an outer diameter of the wide-angle conversion lens is unnecessarily large when using the wide-angle conversion lens with the magnification of 0.8 that meets with the standard of the taking lens and is marketed. As a result, the front face of the taking lens is unnecessarily heavy.
The present invention has been developed in view of the above-described circumstances, and has as its object the provision of a converter in which an auxiliary lens that can be shared by plural lenses is used as a wide-angle conversion lens or a ratio conversion lens corresponding with sizes of an aperture of the taking lens so as to increase the use of one auxiliary lens and thus to more effectively use the auxiliary lens. Moreover, the converter is superior because it is light-weighted as well as has a compact size when using the auxiliary lens as a ratio converter.
In order to achieve the above-described objects, the present invention is directed to a converter comprising: an auxiliary lens that changes focal lengths of a first taking lens and a second taking lens; and an attaching device that attaches and detaching said auxiliary lens to and from a subject side of said first taking lens and said second taking lens, wherein said auxiliary lens serves as a wide-angle conversion lens that reduces an image by expanding a view angle and at the same time forming the image without vignetting within an imaging area when the auxiliary lens is inserted into an optical system of said first taking lens, and said auxiliary lens serves as a ratio conversion lens that reduces the image by expanding the view angle and at the same time forming the image within the reduced imaging area, the image having the view angle with almost the same size as the original imaging area in a case where the imaging area is reduced in relation to an aspect ratio of the imaging area, when the auxiliary lens is inserted into an optical system of said second taking lens.
The converter is characterized in that an aperture of the second taking lens is larger than an aperture of the first taking lens, and when attaching the auxiliary lens to the second taking lens, vignetting occurs within the original image whereas no vignetting occurs within the reduced imaging area.
The converter is further characterized in that the original imaging area has an aspect ratio 16:9, and the reduced imaging area has an aspect ratio 4:3, which are formed by masking a part of both sides of the original imaging area.
According to the present invention, the auxiliary lens can be easily attached and detached to and from the subject side of the taking lens, and the auxiliary lens can be used as a wide-angle conversion lens or a ratio converter corresponding with types (apertures) of taking lenses; thus the auxiliary lens can be shared by other plural lenses as the wide-angle conversion lens or the ratio conversion lens while increasing the use of the auxiliary lens, so the auxiliary lens can be effectively used. Moreover, the auxiliary lens can be light-weighted as well as compact when using the auxiliary lens as a ratio conversion lens, by making the auxiliary lens which is used as a wide-angle conversion lens for a predetermined taking lens to be attached to a taking lens, which can be used as a ratio conversion lens but cannot be used as a wide-angle conversion lens with a larger aperture than that of the taking lens.